<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body {
            display: flex;
            min-height: 100vh;
            margin: 0;
        }

        main {
            margin:
                    auto
        ;
        }
    </style>
</head>
<body>
  <main>center me ,please!</main>
</body>
</html>